Silicone-based defoamers are widely used in processes accompanied by foaming in such areas as the chemical, food, petroleum, papermaking, textile and pharmaceutical industries. Oil compound-type defoamers obtained by mixing a silicone oil such as dimethylpolysiloxane, methylphenylpolysiloxane or methylvinylpolysiloxane with a fine silica powder have been commonly used.
Emulsion-type defoamers obtained by dispersing such an oil compound-type defoamer together with a surfactant in water at the time of actual use, and self-emulsifying defoamers that use both a polyoxyalkylene group-modified organopolysiloxane and an oil compound have also been disclosed (Patent Document 1: JP-A S51-71886; Patent Document 2: JP-B S54-43015; Patent Document 3: JP-B S52-19836; Patent Document 4: JP-B S52-22638; Patent Document 5: JP-B S55-23084). However, their defoaming performances are largely attributable to the oil compound-type defoamer serving as a starting material; in the prior art, the defoaming performance has fallen short. For example, problems that arise include an insufficient initial defoaming performance and a decrease over time in the defoaming performance upon prolonged contact with a foaming liquid. These drawbacks are particularly acute when the foaming liquid is alkaline.
Various solutions have been proposed in order to ameliorate these problems and further improve the defoaming performance. One example is a method in which the silica to be used in the oil compound is rendered hydrophobic beforehand with chlorosilane or the like (Patent Document 6: JP-B S52-31836). Another example is a method in which silica is treated with a nitrogen-containing organosilicon compound (Patent Document 7: JP-B S51-35556). Unfortunately, these solutions are inadequate in terms of performance, with decreases in the defoaming performance occurring over time. Hence, an even further improvement in performance is desired.
Solutions that have been proposed to address these problems include self-emulsifying defoamer compositions (Patent Document 8: JP No. 4232031) and emulsion-type defoamer compositions (Patent Document 9: JP-A 2007-222812) in which the alkali resistance has been increased by mixing and treating, in the presence of an alkaline catalyst, a hydrophobic organopolysiloxane and a fine silica powder surface-treated with an organopolysiloxane. However, these approaches also have drawbacks, including changes over time in the viscosity of the oil compound due to insufficient neutralization in the neutralization step of oil compound production, and the fact that removing the organic acid, inorganic acid or the like used as the neutralizing agent is not easy.
Recently, to address these problems, a method for producing an oil compound that undergoes little change in viscosity over time and from which residual neutralizing agent is easily removed has been disclosed (Patent Document 10: JP-A 2011-104525). This method entails mixing and treating, in the presence of an alkaline catalyst, a mixture of a hydrophobic organopolysiloxane with a fine silica powder, and then carrying out a neutralization reaction using a solid acid such as succinic acid.